uname -o does not exist on mac

[Lassi Kortela]

Here's a small patch to silence a warning in `make` output on systems 
where the `uname` command does not have the `-o` flag.

diff --git a/Makefile.detect b/Makefile.detect
index 10d36545..9694b125 100644
--- a/Makefile.detect
+++ b/Makefile.detect
@@ -19,7 +19,7 @@ else
 # Now we can use uname tests
 uname_s:=$(shell uname)
-uname_o:=$(shell uname -o)
+uname_o:=$(shell uname -o 2>/dev/null)

 # Check for specific platforms
 ifeq ($(uname_o),Msys)